The invention is particularly applicable to production of silicon carbide crystals (herein used to include crystal films) and it will be discussed with particular reference thereto; however, the invention has much broader applications and can be used for other crystals grown by the chemical vapor deposition process.
Semiconductor devices are used in a wide variety of electronic applications. Semiconductor devices include diodes, transistors, integrated circuits, light-emitting diodes and charge-coupled devices. Various semiconductor devices using silicon or compound semiconductors such as gallium arsenic (GaAs) and gallium phosphide (GAP) are commonly used. In order to fabricate semiconductor devices, it is necessary to be able to grow high-quality, low-defect-density single-crystal films with controlled impurity incorporation (with respect to both net concentration and concentration profile) while possessing good surface morphology. In recent years, there has been an increasing interest in research of the silicon carbide semiconductors for use in high temperature, high powered and/or high radiation operating conditions under which silicon and conventional III-V semiconductors cannot adequately function.
Silicon carbide has been classified as a compound semiconductor with potentially superior semiconductor properties for use in applications involving high temperature, high power, high radiation and/or high frequency. Silicon carbide has a number of characteristics that make it highly advantageous for various uses. Such advantages include a wide energy gap of approximately 2.2 to 3.3 electron volts, a high thermal conductivity, a low dielectric constant, a high saturated electron drift velocity, a high breakdown electric field, a low minority carrier lifetime, and a high disassociation temperature. Furthermore, silicon carbide is thermally, chemically and mechanically stable and has a great resistance to radiation damage. In addition, a variety of optical devices, such as light-emitting diodes (LEDs) can be fabricated from silicon carbide and operated at temperatures exceeding 600.degree. C. Despite these many advantages and capabilities of silicon carbide semiconductor devices, large scale commercialization of silicon carbide devices has been slow because of the lack of control over the crystal quality, growth reproducibility, and controlled dopant incorporation into the silicon carbide crystals.
Several properties of SiC contribute to this lack of control. First, it does not melt at reasonable pressures and it sublimes at temperatures above 1800.degree. C. Second, it grows in many different crystal structures, called polytypes. Third, post-growth doping attempts (i.e. diffusion into the crystal from a gas phase species as is used in the silicon industry) are not effective in SiC crystals. Other known post-growth doping techniques (i.e. ion implantation) typically result in considerable crystal damage. Attempts to remove this crystal damage, and therefore improve device performance by post-annealing, commonly result in severe dopant profile redistribution.
Since molten-SiC growth techniques cannot be applied to SiC, two techniques have been developed to grow silicon carbide crystals. The first technique is known as chemical vapor deposition (CVD) in which reacting gases are introduced into a crystal chamber to form silicon carbide crystals upon an appropriate heated substrate. A second technique for growing bulk silicon carbide crystals is generally referred to as the sublimation technique or Lely process. In the sublimation technique, some type of solid silicon carbide material other than the desired single crystal in a particular polytype is used as a starting material and heated until the solid silicon carbide sublimes. The vaporized material is then condensed to produce the desired crystals. Although a large number of crystals can be obtained by either the sublimation method or the epitaxial growth method (CVD), it is difficult to prepare large single crystals of silicon carbide and to control with high accuracy the size, shape, polytype and doping of the silicon carbide crystals.
Silicon carbide crystals exist in hexagonal, rhombohedral and cubic crystal structures. Generally, the cubic structure, with the zincblende structure, is referred to as the .beta.-SiC or 3C-SiC whereas numerous polytypes of the hexagonal and rhombohedral forms are collectively referred to as .alpha.-SiC. The most common .alpha.-SiC polytype is 6H-SiC. Each of the various silicon carbide polytypes have unique electrical and optical properties which give them advantages over the other polytypes in particular applications. For example, the 6H-SiC polytype has a bandgap of about 2.9 electron volts and a hexagonal structure, wherein the 3C-SiC polytype has a lower bandgap of about 2.2 electron volts and has a higher symmetry structure than the 6H-SiC polytype. These property differences lead to advantages for the 6H-SiC polytype in some applications such as a wider bandgap resulting in blue light-emitting diodes and operation at higher temperatures. On the other hand, the 3C-SiC polytype has a higher electron mobility leading to a higher frequency of operation.
SiC polytypes are formed by the stacking of double layers of Si and C atoms. Each double layer may be situated in one of three positions. The sequence of stacking which determines the particular polytype. The stacking direction is called the crystal c-axis which is perpendicular to the basal plane. For 6H-SiC polytypes, the (0001) plane (the Si-face) or (000T) plane (the C-face) is known as the basal plane and for 3C-SiC, the plane (111) is equivalent to the (0001) basal plane.
Many advances have been made in the growing of higher quality 6H-SiC and 3C-SiC crystals having fewer dislocations, stacking faults, microtwins, double positioning boundaries (DPBs), threading dislocations and anti-phase boundaries (APBs). However, with all the advances in growing 3C-SiC and 6H-SiC crystals, the controlled purity and doping of such crystals is limited. Furthermore, controlled and reproducible degenerate doping or very low doping has of yet been unachievable. Dopant incorporation in a grown crystal is known as intentional impurity incorporation and, contaminant incorporation into the crystal is known as unintentional impurity incorporation (i.e. contamination). During the sublimation or CVD crystal growing process, various compounds and/or elements are intentionally and unintentionally incorporated into the crystal. Various techniques of limited success have been used to exclude contaminants from a crystal to produce highly pure crystals. One technique reported for InP and GaAs CVD growth systems is the use of a blocking technique whereby relatively large amounts of a crystal compound are used to block or shield the crystal surface from impurities. Chemistry of The In-H.sub.2 PCl.sub.3 Process by R. C. Clarke, Inst. Phys. Conf.; Ser. No. 45; Chapter 1 (1979) 19-27 and Doping Behavior of Silicon and Vapor Growth III-V Epitaxial Films by H. P. Pogue and B. M. Quimlich, J. Crystal Growth 31 (1975) 183-89. Although the blocking technique reduces contaminant incorporation into a crystal, the required use of large amounts of a crystal growing compound to effect blocking has adverse effects on crystal quality and surface morphology. In addition to the problems associated with contaminant exclusion, the controlled intentional doping of crystals at relatively low concentrations has of yet been unachievable and unreproducible. Furthermore, techniques to control and/or reproduce sharp dopant concentration profiles within crystals (i.e. from p-type to n-type, degenerate to lightly doped) are also unavailable.
A variety of prior art techniques have been developed, and tried, in an attempt to control the impurity incorporation in the growing SiC films. For example, molecular beam epitaxy (MBE) utilizes an ultrahigh vacuum system and the growth of the crystal occurs by using a stream of molecules which is formed into a beam and focused onto a heated substrate. This technique does offer some amount of control over dopant-concentration profiles. However, the low rate of growth is a major drawback and poses several problems. First, commercialization is not practical because of the very low growth rate. Second, the impurity incorporation is still limited by the purity of the source gas and cleanliness of the growth reactor. Furthermore, the problem of impurity incorporation is exacerbated by the very slow growth rate.
Another technique for doping SiC is the use of ion implantation, which is a post-growth doping technique used to introduce the desired dopants. This method produces a large amount of damage to the crystal structure and typically requires a post-anneal step to reduce the high density of defects (which greatly affects device quality) generated by this technique. Furthermore, as a result of the unusually high temperatures (&gt;1800.degree. C.) needed for the only partially effective annealing of SiC crystals, the dopant concentration changes by a factor of four (4.times.) for p-type and is lost via out-diffusion for the n-type dopants.
When degenerately doped layers are desired, such as the case when metal contact layers are needed, obtaining degenerate p-type via CVD is limited by problems such as gas phase nucleation (from an excessively high concentration of p-type source gas needed) which results in very poor film morphologies.
As a result of the inadequate techniques available to control dopant and/or contaminant incorporation into crystals grown in a CVD process, there is a demand for a method to selectively exclude impurities from a CVD grown crystal.